The outer shell of a steam turbine is generally called the outer casing. Within the outer casing is typically an inner casing that supports stationary vane rows and a rotor that supports rotating blade rows, wherein the space formed between the inner casing and the rotor is an expansion flow path. As the rotor, inner casing, and outer casing are exposed to different temperatures and further may be constructed of different materials with differing thermal expansion coefficients, expansion rates of these components may differ. As a result, it is typical to tolerate large axial clearances between blade rows and vane rows in order to take into account this differing thermal expansion, which is most pronounced when the steam turbine is exposed to different loads or else changes operating stated from standby mode to running mode.
However, larger than required axial clearances between blade rows and vane rows may result in a reduction of the efficiency of the turbine. This is, however, preferably to the contrary where contact is made between components. The problems caused by large axial clearances can in part be addressed through measurement. For example, measuring the clearance between a plurality of turbine blade row shroud segments can be done using proximity sensors. This allows an operator to take action if a critical clearance condition occurs. Other than the discussed measurement means, other known blade row tip clearance measurement methods used in gas turbines maybe adapted for use in steam turbines. Such measurement methods include fibre optic laser Doppler distance sensors, as well as other sensors such as capacitive probes, inductive probes, optical measurement systems based on triangulation, optical coherence tomograph and time-of-flight measurements. While these solutions may provide a desirable means of alerting an operator to the approach of a potentially undesirable condition, the corrective action, which typically may result in a change in load, typically requires the steam turbine to be operated away from a desired operating point.
Another solution may include a method and an arrangement for aligning an inner casing of a steam turbine by means of adjustable mounts. During operation of the steam turbine the axial clearance between the inner casing and rotor is measured and then the axial clearance between the rotor and the inner casing is adjusted.